The present invention relates to a mooring system for tanker vessels that employs a vessel mounted turret in combination with a single anchor chain, three leg mooring arrangement to automatically accommodate movement of the vessel due to environmental forces, provide a removable connection to a subsea oil riser, and be easily adapted to varying water depths.
FPSOs are Floating Production, Storage and Offloading vessels which are employed to temporarily store oil received from offshore subsea and dry surface wells (or in the case of an FSO, pumped from offshore production facilities) until an oil tanker anives to receive the oil from the FPSO. Typically, an FPSO is itself an oil tanker that has been modified to receive the oil directly from one or more subsea risers (flexible pipelines), and be moored in a fixed position near the offshore oil wellheads or production facilities. The mooring systems employed to hold the FPSOs in place must be specially designed for a number of reasons. In particular, the mooring system must not interfere with one or more subsea oil risers that are employed to transfer oil from the offshore production facility to the FPSO. Additionally, the mooring system must be designed to withstand adverse environmental conditions, including rough seas and strong winds, since offshore production facilities are typically located in areas subject to such conditions.
The foregoing requirements have resulted in the design of a number of different mooring arrangements for FPSOs. The most popular design is known as a single point mooring system in which only the bow (or stern) of the FPSO is attached to the mooring, thus allowing the FPSO to pivot or "weathervane" about a single axis of rotation in response to wind or water currents. The most popular of the single point mooring systems is the turret mooring system. Previous turret mooring systems have consisted of a number of mooring lines that are attached to a rotatable turret. The risers enter the rotatable turret centrally inside a turret chain attachment-point radius. The turret mooring may be attached to the sea bed by 5 to 20 mooring lines in a radial or clustered pattern. A number of these turret mooring systems have been built so that they can be disconnected before severe storms approach. The turret mooring is then dropped or lowered into the sea from the FPSO while the turret turntable is still attached to the risers and the anchor chains. These systems are complex and expensive to build, and they are not built for multiple relocations at low cost. The weathervaning action also presents problems if the water and/or wind currents are such that the FPSO pivots more than 360 degrees about its starting position, thereby twisting the oil riser(s) and the mooring chains. Hence, the turret moorings are made such that they can rotate around a single point of rotation on bearings.